This invention relates to electrostatography and more particularly to improved electrostatographic developing materials fixing systems and the use thereof.
In electrostatography, more specifically recited in U.S. Pat. No. 2,297,691, a uniform electrostatic charge is placed on a photoconductive insulating layer, selectively exposed, and the resulting latent electrostatic image is developed to provide a visible reproduction of an original by depositing on the image a finely divided electroscopic marking material referred to in the art as "toner". Toner is normally attracted to those areas of the layer which retain a charge thereby forming a toner image corresponding to the electrostatic latent image. The image so produced may be transferred to a support surface or otherwise processed. The image may then be permanently affixed to the support surface employing conventional fixing methods such as heating or application of a suitable solvent.
Toner alone or in combination with a suitable carrier and additives, where appropriate, may be applied employing a number of development techniques among which are cascade, more fully defined in U.S. Pat. No. 2,618,552 to E. N. Wise; magnetic brush, more fully defined in U.S. Pat. No. 2,874,063; powder cloud, more fully defined by Carlson in U.S. Pat. No. 2,221,776; or touch-down development, as disclosed by Gundlach in U.S. Pat. No. 3,166,432; among others.
When fixing the final image by application of heat, problems in adapting such a technique to high-speed machines in view of the energy required to raise the temperature of the toner to the desired level, charring or combustion, and specifically adapting toner materials to these parameters for use in high-speed electrostatographic copying machines has resulted in the requirement for additional complex machinery and process techniques. The development of an appropriate toner material which will fuse under high-speed machine conditions and avoid blocking or caking, process readily and exhibit the appropriate triboelectric properties under changes in the ambient humidity has become a rather critical and demanding art in and of itself. Other characteristics and properties that must be controlled or eliminated in a desirable toner include the effects of impaction on the triboelectric properties the abrasive nature of the toner, and bead sticking which is the adherence of carrier beads to a reusable photoconductor surface.
Toner particles are usually comprised of thermoplastic resins selected to have melting points significantly above any ambient temperature that might be encountered during electrostatic deposition. In addition to the developing powder or toner materials described in U.S. Pat. No. 2,297,691 a number of additional toner materials have been developed especially for use in the newer development techniques including the cascade development technique described above. Generally speaking, these new toner materials have comprised various improved resins mixed with different pigments such as carbon black and other colorants. Some examplary patents along this line include U.S. Pat. No. 2,659,670 to Copley which describes a toner resin as rosin modified phenyl formaldehyde, U.S. Pat. No. Re. 25,136 to Carlson which describes an electrostatographic toner employing a resin of polymerized styrene and U.S. Pat. No. 3,079,342 to Insalaco describing a plasticized copolymer resin in which the comonomers are styrene and a methacrylate selected from the group consisting of butyl, isobutyl, ethyl, propyl, and iso-propyl.
Generally, these toners have been prepared by thoroughly mixing a heat softened resin and a colorant to form a uniform dispersion as by blending these ingredients in a rubber mill or the like and then pulverizing this material after cooling to form it into small particles. These toners, though they result in excellent image reproductions, do exhibit some disadvantages such as a rather wide range of particle sizes and the ability of the colored resin to be sufficiently pliable for high-speed pulverizing which results in an even wider range of particle sizes during pulverization. Other requirements of electrostatographic developers or toners including the requirements that they be stable in storage, non-agglomerative, have the proper triboelectric properties for developing and have a low melting point for heat fusing are only compounded by the additional requirements imposed by this toner forming process. It is, therefore, found that some developer materials, such as those containing toner particles made from low molecular weight resins though possessing desirable properties such as proper triboelectric characteristics are unsuitable because they tend to cake, bridge, and agglomerate during handling and storage. Another significant problem in the case of conventional toners is the high energy requirements for heat fusing sources employed with these toners.
Electrostatographic developer materials which are pressure fixable have been considered in view of the above stated difficulties. However, it is found that the toner requirements for good machine performance tend to be diametrically opposed to the requirements for pressure fixing. That is, low toner impaction requires a high toner softening temperature and good mechanical strength while pressure fixing requires softening and viscous flow at room temperature. In addition, one of the problems with potential pressure fixable toners is the need to gently handle these materials prior to pressure fusion to paper or other suitable support medium so that these materials will not prefuse and cause impaction in the development chamber. Therefore, a balance must generally be made between a material which will pressure fix onto paper at low pressure but not yet impact in the development chamber. A major cause of such prefusion is the abrasive action of the tumbling carrier beads on the toner both in normal cascade development and magnetic brush development.
Electrostatographic toner materials which are capable of pressure fixing are desirable and advantageous since unencapsulated materials which undergo cold flow tend to form tacky images on the copy sheet which often offset to other adjacent sheets. Toner particles containing unencapsulated materials which undergo cold flow, tend to bridge, cake, and block during production and in the shipping container as well as in the electrostatographic imaging machine. Of course, the toner material should be capable of accepting a charge of the correct polarity such as when brought into rubbing contact with the surface of carrier materials in cascade, magnetic brush, or touch-down development systems. Further, it is found that some toner materials which possess many properties as aforementioned which wound ordinarily be desirable in electrostatographic toners dispense poorly and cannot be used in automatic copying and duplicating machines. Still other toners dispense well but form images which are characterized by low density, poor resolution, or high background. Still other toners are suitable for processes where electrostatic transfer is employed.
In addition to pressure fix systems, other systems of a so-called hybrid nature have been employed such as pressure-vapor fix system wherein both pressure and a solvent vapor are applied to accomplish fixing or other combinations of heat, pressure and solvent fixing. Because of the energy requirements associated with heat fusing and the handling of an environmental problem associated with vapor fusing there is a demonstrated need for improved toner fixing systems.
It is, therefore, an object of this invention to provide a toner fixing system which is devoid of the above noted deficiencies.
Another object of this invention is to provide a toner fixing system which employs substantially reduced energy levels.
Again another object of this invention is to provide a novel toner system.
Yet another object of this invention is to provide a toner which is stable at toner fusing conditions in high-speed copying and duplicating machines.
Still another object of this invention is to provide an impaction resistant toner material.
Again, another object of this invention is to provide a toner material which is resistant to smearing, agglomeration, and may be fused readily with less heat energy.
Yet still another object of this invention is to provide a toner which reduces mechanical abrasion of electrostatic imaging surfaces and is effective at low initial electrostatic surface potentials to provide dense toner images.
Again another object of this invention is to provide a toner which allows toner fixing at higher rates with less pressure.
These and other objects of the present invention are accomplished generally speaking by providing a photodegradable toner system. More specifically novel toner compositions are provided which comprise photodegradable polymers so that when employed in a xerographic process these toners may be applied to render a latent electrostatic image visible employing conventional techniques and thereafter be fixed employing a hybrid fixing system which utilizes the photodegradability of the toner material itself. The toner of the instant invention is, therefore, applied either by itself or in connection with a carrier in the form of a developer during the developing step in a conventional electrophotographic process. Thereafter it is fixed e.g., by exposure to activating radiation which degrades the photodegradable polymer relieving the stress on the polymer and thereby allowing it to become affixed to the substrate on which it is deposited, followed by the application of pressure. In employing such a hybrid e.g., a combination of light and then pressure activated stress relief fixing system, substantially lower energy levels may be employed that is found in conventional pressure fix systems and in addition the solvent vapor step with its accompanying drawbacks is eliminated as found in other hybrid systems. Although pressure followed by light exposure may be employed to obtain satisfactory fixing the sequence of light exposure followed by application of pressure yield more desirable results. Photo fixing has certain advantages to normal heat fusing among which are the utilization of lower energy requirements, less fire hazard in electrophotographic imaging machines and the possibility of employing less complicated types of paper stocks. Polymer degradation may, therefore, be employed of the free radical type which involves decomposition by random chain scissions as well as depolymerization to monomer to effect efficient hybrid flash-pressure fixing. This system is limited to random scissions with quantum efficiencies .ltoreq. 35. The present photo-oxidant process is ionic and depolymerization to monomer occurs with higher quantum efficiency. This process may either be accomplished by providing a toner material wherein a solvent which effects the stress relaxation is formed in situ by photochemical process or alternatively a photodegradable polymer is allowed to degrade to lower "mers" which will simultaneously lower the viscosity and plasticize undegraded polymer thereby enhancing fixability in the hybrid fixing mode.